[AIRFLOW-5085] we always pass the default branch name to the build (#5699)
[AIRFLOW-5085] we always pass the default branch name to the build (#5699)

TRAVIS_BRANCH is set to TAG when TAG build runs. We should alwayss
use branch and we already have our current branch in
hooks/_default_branch.sh and we can use it from there.

This seems to be the only way as TRAVIS does not pass the branch
in any variable - mainly because we do not know what branch we
are in when building a TAG build
apache/airflow,Fokko/incubator-airflow,danielvdende/incubator-airflow,mistercrunch/airflow,danielvdende/incubator-airflow,wileeam/airflow,wileeam/airflow,spektom/incubator-airflow,wileeam/airflow,mtagle/airflow,mrkm4ntr/incubator-airflow,mtagle/airflow,cfei18/incubator-airflow,lyft/incubator-airflow,mrkm4ntr/incubator-airflow,apache/incubator-airflow,dhuang/incubator-airflow,cfei18/incubator-airflow,danielvdende/incubator-airflow,nathanielvarona/airflow,apache/airflow,Acehaidrey/incubator-airflow,Acehaidrey/incubator-airflow,lyft/incubator-airflow,mistercrunch/airflow,nathanielvarona/airflow,wooga/airflow,DinoCow/airflow,airbnb/airflow,mtagle/airflow,wileeam/airflow,spektom/incubator-airflow,apache/incubator-airflow,bolkedebruin/airflow,apache/airflow,bolkedebruin/airflow,nathanielvarona/airflow,dhuang/incubator-airflow,Fokko/incubator-airflow,airbnb/airflow,Acehaidrey/incubator-airflow,apache/airflow,danielvdende/incubator-airflow,danielvdende/incubator-airflow,Acehaidrey/incubator-airflow,apache/airflow,dhuang/incubator-airflow,wooga/airflow,DinoCow/airflow,nathanielvarona/airflow,nathanielvarona/airflow,DinoCow/airflow,cfei18/incubator-airflow,cfei18/incubator-airflow,apache/incubator-airflow,wooga/airflow,sekikn/incubator-airflow,mrkm4ntr/incubator-airflow,Fokko/incubator-airflow,lyft/incubator-airflow,bolkedebruin/airflow,apache/airflow,airbnb/airflow,sekikn/incubator-airflow,mistercrunch/airflow,cfei18/incubator-airflow,sekikn/incubator-airflow,wooga/airflow,mistercrunch/airflow,Acehaidrey/incubator-airflow,danielvdende/incubator-airflow,dhuang/incubator-airflow,Fokko/incubator-airflow,mtagle/airflow,Acehaidrey/incubator-airflow,bolkedebruin/airflow,airbnb/airflow,spektom/incubator-airflow,lyft/incubator-airflow,sekikn/incubator-airflow,mrkm4ntr/incubator-airflow,cfei18/incubator-airflow,apache/incubator-airflow,DinoCow/airflow,bolkedebruin/airflow,spektom/incubator-airflow,nathanielvarona/airflow